1. Field of the Invention
The present invention relates to a sheet processing apparatus which holds a doubled up booklet and subjects the booklet to predetermined processing, and the invention also relates to an image forming apparatus having the sheet processing apparatus.
2. Description of the Related Art
Conventionally, if a batch of sheets (twenty or more sheets) are folded in two by one operation, a spine (back) portion is apparently curved, and a completed booklet has insufficient folded portion. Therefore, there is provided an apparatus which flattens the spine of a batch of sheets, thereby sharply folding the sheets to enhance the folding performance as described in a specification of U.S. Patent Application Publication No. 2003/0031532.
FIG. 18A illustrates a configuration of the apparatus. A booklet H which was folded by one operation is nipped by a pair of belts 51a and 51b and conveyed, the booklet H abuts against a tip end stopper 52a and then, the booklet H is held by holding portions 54 and 55. If the holding operation is completed, a pressure roller 59 presses a back of the booklet H, and sharply folds the back while moving. FIG. 18B illustrates a state where the booklet H is held by the holding portions 54 and 55.
Conventionally, to enhance storage performance of a saddle stitched booklet, a unit apparatus for forming holes through which the booklet is bound with a string is provided by Japanese Patent Laid-open No. 2003-228205.
FIG. 19A illustrates a configuration of the punch apparatus. The saddle stitched booklet H is nipped by a belt 61 and conveying rollers 62 and 63 and conveyed, the booklet H abuts against a tip end stopper 64 and thereafter, a holding portion 65 is lowered and the booklet H is held. If the holding operation is completed, a punching portion 66 punches holes in the booklet H. FIG. 19B illustrates a state where the booklet is held by the holding portion 65.
According to the conventional configuration, however, since the booklet H is nipped and conveyed and is abutted against the tip end stopper 52a as illustrated in FIG. 19B, there is an adverse possibility that several sheets from the outermost sheet to inner sheets near a back H1 of the booklet H which abuts against the tip end stopper 52a sag. In this case, even if the booklet is held thereafter, the sag remains. This phenomenon outstandingly appears when a friction coefficient of a surface of the outermost sheet of the booklet is high (no image) and a friction coefficient of the other surface is low (entire surface is color photograph). The phenomenon also appears when the outermost sheet of the booklet is thinner than center sheets and the outermost sheet has lower rigidity. Large creases as illustrated with portions C in FIG. 20A are generated due to this sag when the back H1 of the held booklet H is sharply folded. When holes are formed near the back H1 of the booklet H, since positions of holes are deviated as illustrated in FIG. 20B, quality of the booklet is deteriorated.
To eliminate the sag, it is conceived to stop the conveying motion at the instant when the booklet abuts against the tip end stopper or to stop the booklet without providing the tip end stopper. However, in the former method, if the booklet skew fed, this skew feeding can not be corrected. To correct the skew feeding, even after one end of the booklet abuts against the tip end stopper, the conveying motion of the booklet for allowing the other end to abut against the tip end stopper must be continued. As a result, a sag is generated. In the latter method, skew feeding of the booklet can not be corrected of course, and since stopping precision depends on precision of a sending amount of the conveying portion, if slip is generated between the booklet and the conveying portion, the stopping precision is largely deteriorated. It seems to be possible to push a rear end of a booklet in which a sag is generated and convey the booklet without nipping and conveying the booklet. However, if a folded portion of the booklet which was folded in two is not nipped, the folded portion of the booklet swells, and then, the booklet cannot proceed between the holding portions before lowering the holding portions.